Hermetically sealed secondary batteries containing a nonaqueous electrolyte, such as lithium ion secondary batteries and nickel hydride batteries have been used in recent years as portable power supplies for personal computers and handheld devices and also as power supplies for driving vehicles, because of their light weight and high energy density compared with existing batteries. In particular, lithium ion secondary batteries, which are lightweight and achieve a high energy density, are advantageously used as high-power sources of electricity for driving vehicles such as electric cars and hybrid cars.
When such sealed secondary batteries are used under harsh conditions such as in on-board applications (specifically, when stored a long time in a high-temperature environment or repeatedly charged and discharged at a high power density), the battery internal pressure can be rose unnecessarily on account of the volumetric expansion of gases remaining within the battery case, the formation of gases due to self-discharge of the battery, and increases in the vapor pressure of the electrolyte solution. Also, in lithium ion secondary batteries that use a carbon material, particularly a graphite material, as the component of negative electrode, the manufacturing process (e.g., at the time of preliminary charging which serves as an initial charging step) are sometimes accompanied by gas formation; the gases that form during such initial charging may likewise cause a rise in the battery internal pressure. Such a rise in the battery internal pressure is undesirable because it may cause a pressure sensing-type current interrupt device (CID) or a gas release valve to malfunction during normal use, or it may became a major factor lowering the long-term reliability of the airtightness of junctional region such as gas release valves and external connection terminals.
Up until now, such problems have been dealt with using such measures as placing the interior of the battery case during and after preliminary charging in a pressure-reduced state following electrolyte injection so as to remove gases that form during preliminary charging, and sealing the battery case (typically, the electrolyte injection hole) (see Patent Literature 1 and 2). In addition, Patent Literature 3 discloses art for injecting a nonaqueous electrolyte into a pressure-reduced battery case.